A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
The term “patterning device” as here employed should be broadly interpreted as referring to device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The term ‘light valve’ can also be used in this context. Generally, the pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device. An example of such a patterning device is a mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern of the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved to the beam if so desired.
Another example of a patterning device is a programmable mirror array. One example of such an array is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that, for example, addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind. In this way, the beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The requires matrix addressing can be performed using suitable electronics. In both of the situations described hereabove, the patterning device can comprise one or more programmable mirror arrays. In the case of the programmable mirror array, the support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning device may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time.
In a known manufacturing process using a lithographic projection apparatus, a pattern (e.g. mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are desired, then the whole procedure, or a variant thereof, will need to be repeated for each new layer. It is desirable to ensure that the overlay (juxtaposition) of the various stacked layers is as accurate as possible. For this purpose, a small reference mark is provided at one or more positions on the wafer, thus defining the origin of a coordinate system on the wafer. Using optical and electronic devices (refereed to hereinafter as ‘alignment system’), this mark can then be relocated each time a new layer has to be juxtaposed on an existing layer, and can be used as an alignment reference. Eventually, an array of devices will be present on the substrate (wafer). These devices are than separated from one another by a technique such as dicing or sawing, when the individual devices can be mounted on a carrier, connected to pins, etc.
For the sake of simplicity, the projection system may hereinafter be referred to as the ‘lens’. However, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a ‘lens’. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such ‘multiple stage’ devices, the additional tables my be used in parallel or preparatory steps may be carried out on one or more tables, while one or more other tables are being used for exposures. This technique is called dual stage lithography and is known per se in the art.
In a lithographic apparatus, the size of features that can be imaged on the substrate is somewhat limited by wavelength of the projection radiation. To produce integrated circuits with a higher density of devices, and hence higher operating speeds, it is desirable to be able to image smaller features. While most current lithographic projection apparatus employ ultraviolet light generated by mercury lamps or excimer lasers, it has been proposed to use shorter wavelength radiation in the range 5 to 20 nm, especially around 13 nm. Such radiation is termed extreme ultraviolet (EUV) or soft X-ray and suitable sources include, for example, laser-produced plasma sources, discharge plasma sources or synchrotron radiation from electron storage rings.
In a discharge plasma source, for example, a discharge is created in between electrodes, and a resulting partially ionized plasma may subsequently be caused to collapse to yield a very hot plasma that emits radiation in the EUV range. Sn, Li and Xe plasmas may be used to radiate in the extreme UV (EUV) range around 13.5 nm.
In addition to EUV radiation, radiation sources used in EUV radiation systems may generate contaminant material that is harmful for the optics and the working environment in which the lithographic process is carried out. Such a radiation system typically comprises a pair of electrodes to which a voltage difference can be applied. In addition, plasma is produced, for example, by a laser beam that is targeted to for example, one of the electrodes. Accordingly, a discharge will occur between the electrodes, which causes a so-called pinch in which EUV radiation is produced. In addition to this radiation, the discharge source typically produces debris particles, which can be all kinds of micro particles varying in size from atomic to complex particles, which can be both charged and uncharged.
It is desired to shield the optical system that is arranged to condition the beam of radiation coming from the EUV radiation source from this debris. Shielding of the optical system is done with a contamination preventing system for preventing material (debris) emanating from the source from propagating with the EUV radiation into the lithography apparatus.
Another product of the source is heat, which causes the contamination prevention system to heat up. This heating up may especially be the case if the size of the contamination prevention system is enlarged to collect radiation over larger collection angles of the source, and is unwanted because heating up may cause malfunction of the contamination prevention system.